1. Field of the Invention
The present invention relates to a projection display device that is provided with a reflective image display element, and more particularly, to prism optics for both guiding illuminating light to an image display element and guiding the light that is reflected from the image display element to projection optics.
2. Description of the Related Art
The development of projection display devices has made rapid strides in recent years, and demand is growing for more compact and lower-cost projectors. This demand is accompanied by a growing need for more compact devices that for use in projection display devices. Meeting this demand calls for the development of more compact peripheral parts, and in particular, for more compact optics that accompany the reduction of the size of these devices, so that the overall size of a projection display device can be decreased. Recent years have seen the development of the time-division image projection device having a basic structure that extracts and projects the colors red, green, and blue in time divisions. For example, as disclosed in Japanese Patent Laid-Open Publication No. 78550/98, a DMD projector is known that uses a digital mirror device (hereinbelow abbreviated “DMD”), in which several hundred thousand individual mirror elements having various controllable inclinations are attached over semiconductor memory cells, whereby the inclination of each mirror element is manipulated to control the reflection state and thus form an image.
As shown in FIG. 1, this DMD projector is configured such that light that is emitted from light source 101 is reflected by reflecting mirror 102 and then condensed by condensing lens 103 to one point of color wheel 104 in which color filters that selectively pass red, green and blue light are combined. The light that is thus passed is made uniform by integrator rod 105, irradiated as a parallel flux by optics 109 such as a relay lens onto DMD 107 by way of TIR prism (total internal reflection prism) 106, and the image light, which is the light reflected by DMD 107, is projected by way of zoom projection lens 108 onto a screen. In this case, TIR prism 106 is configured such that two prisms having different indices of refraction are bonded together, whereby the illumination light that is irradiated onto the bonded surface is reflected and irradiated upon DMD 107, and light that is reflected from DMD 107 is transmitted through the bonded surface and emitted from prism 106 to projection lens 108. Regarding the structure of this TIR prism, the configuration disclosed in Japanese Patent No. 3060049 or Japanese Patent No. 3065058 may be adopted.
The TIR prism in this DMD projector of the prior art has a configuration in which two prisms are bonded for total reflection as well as transmission of illumination light. In addition, this TIR prism requires the previously described light-condensing relay lens in the stage preceding the TIR prism. This arrangement results in an increase in space for mounting the optical element portion that is disposed in the space between the integrator rod and the projection lens and therefore poses a serious obstacle to creating a compact projection display device. The same problem exists in a projection display device that employs a reflective liquid crystal panel as the image display element.